Methods of forming SiC MOSFETs with high inversion layer mobility

ABSTRACT

Methods of forming an oxide layer on silicon carbide include thermally growing an oxide layer on a layer of silicon carbide, and annealing the oxide layer in an environment containing NO at a temperature greater than 1175° C. The oxide layer may be annealed in NO in a silicon carbide tube that may be coated with silicon carbide. To form the oxide layer, a preliminary oxide layer may be thermally grown on a silicon carbide layer in dry O 2 , and the preliminary oxide layer may be re-oxidized in wet O 2 .

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/486,752, filed on Jul. 14, 2006 now U.S. Pat. No. 7,727,904 whichclaims the benefit of and priority to U.S. Provisional PatentApplication No. 60/717,953, filed Sep. 16, 2005, the disclosures ofwhich are hereby incorporated herein by reference as if set forth intheir entireties.

STATEMENT OF GOVERNMENT INTEREST

The present invention was made, at least in part, with support fromUnited States Air Force contract number FA8650-04-2-2410 and ARL/MTOcontract number W911NF-04-2-0022. The Government has certain rights inthis invention.

FIELD OF THE INVENTION

This invention relates to methods of fabricating power devices and theresulting devices, and more particularly to silicon carbide powerdevices and methods of fabricating silicon carbide power devices.

BACKGROUND

Power semiconductor devices are widely used to carry large currents andsupport high voltages. Modern power devices are generally fabricatedfrom monocrystalline silicon semiconductor material. One widely usedpower device is the power Metal Oxide Semiconductor Field EffectTransistor (MOSFET). In a power MOSFET, a control signal is supplied toa gate electrode that is separated from the semiconductor surface by anintervening insulator, which may be, but is not limited to, silicondioxide. Current conduction occurs via transport of majority carriers,without the presence of minority carrier injection that is used inbipolar transistor operation. Power MOSFETs can provide an excellentsafe operating area, and can be paralleled in a unit cell structure.

As is well known to those having skill in the art, power MOSFETs mayinclude a lateral structure or a vertical structure. In a lateralstructure, the drain, gate and source terminals are on the same surfaceof a substrate. In contrast, in a vertical structure, the source anddrain are on opposite surfaces of the substrate.

One widely used silicon power MOSFET is the double diffused MOSFET(DMOSFET) which is fabricated using a double-diffusion process. In thesedevices, a p-base region and an n+ source region are diffused through acommon opening in a mask. The p-base region is driven in deeper than then+ source. The difference in the lateral diffusion between the p-baseand n+ source regions forms a surface channel region.

Recent development efforts in power devices have also includedinvestigation of the use of silicon carbide (SiC) devices for powerdevices. Silicon carbide (SiC) has a combination of electrical andphysical properties that make it attractive for a semiconductor materialfor high temperature, high voltage, high frequency and high powerelectronic devices. These properties include a 3.0 eV bandgap, a 4 MV/cmelectric field breakdown, a 4.9 W/cm-K thermal conductivity, and a2.0×10⁷ cm/s electron drift velocity.

Consequently, these properties may allow silicon carbide power devicesto operate at higher temperatures, higher power levels and/or with lowerspecific on-resistance than conventional silicon-based power devices. Atheoretical analysis of the superiority of silicon carbide devices oversilicon devices is found in a publication by Bhatnagar et al. entitled“Comparison of 6H—SiC, 3C—SiC and Si for Power Devices”, IEEETransactions on Electron Devices, Vol. 40, 1993, pp. 645-655. A powerMOSFET fabricated in silicon carbide is described in U.S. Pat. No.5,506,421 to Palmour entitled “Power MOSFET in Silicon Carbide” andassigned to the assignee of the present invention.

4H—SiC Power DMOSFETs have the potential to offer significant advantagesover conventional high voltage Si power switches. Unfortunately,however, it may be difficult to thermally grow an acceptable gate oxidefor these devices. Much effort has been focused on reducing theinterface trap density (D_(IT)) at the SiC/SiO₂ interface in order toincrease the channel mobility (μ_(CH)) of the devices. Nitric Oxide (NO)anneals at 1175° C. have increased the μ_(CH) from single digits to ˜30cm²/Vs. See, e.g., G. Y. Chung, et al., IEEE Electron Dev. Let. 22, 176(2001). Researchers have demonstrated even higher channel mobility (˜150cm²/Vs) by oxidizing in an environment containing metallic impurities.See, e.g., U.S. Pat. No. 6,559,068 . However, such a process may resultin significant oxide contamination, may provide an uncontrolledoxidation rate (t_(OX)>1500 Å), and/or may be incompatible with hightemperature processing steps such as may be used for ohmic contactanneals.

SUMMARY

Methods of forming an oxide layer on silicon carbide according to someembodiments of the invention include thermally growing an oxide layer ona layer of silicon carbide, and annealing the oxide layer in anenvironment containing NO at a temperature greater than 1175° C.

Annealing the oxide layer may include annealing the oxide layer in anenvironment containing NO at a temperature between about 1200° C. andabout 1600° C. In particular embodiments, annealing the oxide layer mayinclude annealing the oxide layer in an environment containing NO at atemperature of about 1300° C. Furthermore, the oxide layer may beannealed for about 2 hours.

The methods may further include placing the oxide layer on the siliconcarbide layer in a silicon carbide tube, and annealing the oxide layermay include annealing the oxide layer in the silicon carbide tube. Thesilicon carbide tube may include a tube of silicon carbide having asilicon carbide coating thereon. The silicon carbide coating on thesilicon carbide tube may include a silicon carbide coating deposited bychemical vapor deposition on the silicon carbide tube.

Thermally growing the oxide may include thermally growing the oxide inthe presence of metallic impurities. In particular, thermally growingthe oxide may include thermally growing the oxide in the presence ofalumina including the metallic impurities. Thermally growing the oxidelayer may include thermally growing the oxide layer to a thickness ofbetween about 500 Å and 900 Å. Thermally growing the oxide layer mayinclude thermally growing a preliminary oxide layer on the siliconcarbide layer in dry O2 at a temperature of about 1200° C., andre-oxidizing the preliminary oxide layer in wet O2 at a temperature ofabout 950° C.

The silicon carbide layer may include an epitaxial layer of 4H p-typesilicon carbide having an off-axis orientation that may be tilted atabout 8° from a (0001) plane.

Methods of forming a silicon carbide MOS structure according to someembodiments of the invention include thermally growing an oxide layer ona layer of silicon carbide, annealing the oxide layer in an environmentcontaining NO at a temperature greater than 1175° C., and forming a gateelectrode on the oxide layer.

The methods may further include forming a gate contact on the oxidelayer, the gate contact including polysilicon and/or a metal.

The silicon carbide layer may include a region of p-type siliconcarbide, and the methods may further include forming an n-type region inthe p-type silicon carbide region. Thermally growing the oxide layer mayinclude thermally growing the oxide layer on the p-type silicon carbideregion and at least partially on the n-type region.

The p-type silicon carbide region may include a p-type epitaxial layer,and the n-type region may include an n-type source region. The methodsmay further include forming an n-type drain region in the p-typeepitaxial layer that is spaced apart from the n-type source region andthat defines a channel region between the source region and the drainregion. Thermally growing the oxide layer may include thermally growingthe oxide layer on the channel region.

The methods may further include forming ohmic contacts on the n-typesource region and the n-type drain region, and annealing the ohmiccontacts on the n-type source region and the n-type drain region at atemperature of at least about 500° C. The channel region may have achannel mobility of at least about 40 cm²/Vs at room temperaturefollowing the ohmic contact anneal.

The p-type silicon carbide region may include an implanted p-type wellregion, and the n-type region may include an n-type source region. Themethods may further include forming the implanted p-type well regionadjacent an n-type JFET region that extends from a surface of thestructure to a drift region disposed beneath the p-type well region, andthermally growing the oxide layer may include thermally growing theoxide layer on a channel region extending in the p-type well regionbetween the source region and the JFET region.

The methods may further include forming an ohmic contact on the n-typesource region, and annealing the ohmic contact on the n-type sourceregion at a temperature of at least about 500° C. The channel region mayhave a channel mobility of at least about 35 cm²/Vs at room temperaturefollowing the ohmic contact anneal.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate certain embodiment(s) of theinvention. In the drawings:

FIG. 1 is a cross sectional illustration of a lateral MOSFET accordingto some embodiments of the invention;

FIG. 2 is a cross sectional illustration of a vertical power MOSFETaccording to some embodiments of the invention;

FIG. 3A is a graph of measured and theoretical values of capacitanceversus voltage for MOSFET devices formed in accordance with conventionaltechniques;

FIGS. 3B-3C are graphs of measured and theoretical values of capacitanceversus voltage for MOSFET devices formed in accordance with someembodiments of the invention;

FIG. 4 is a graph of interface state density (D_(IT)) versus energylevel from the conduction band for MOSFET devices formed in accordancewith some embodiments of the invention as well as MOSFET devices formedin accordance with some conventional techniques;

FIG. 5 is a graph of channel mobility versus gate voltage measured atroom temperature for lateral MOSFET devices formed in accordance withsome embodiments of the invention as well as MOSFET devices formed inaccordance with conventional techniques;

FIG. 6 is a graph of channel mobility versus gate voltage measured atvarious temperatures for lateral MOSFET devices formed in accordancewith some embodiments of the invention;

FIG. 7 is a graph of channel mobility versus gate voltage at roomtemperature for implanted-channel MOSFET devices formed in accordancewith some embodiments of the invention as well as MOSFET devices formedin accordance with some conventional techniques; and

FIG. 8 is a plot of SIMS analysis of a MOS structure including an oxideformed according to some embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. It will be understood that,although the terms first, second, third etc. may be used herein todescribe various elements, components, regions, materials, layers and/orsections, these elements, components, regions, layers and/or sectionsshould not be limited by these terms. These terms are only used todistinguish one element, component, region, layer, material or sectionfrom another element, component, region, layer, material or section.Thus, a first element, component, region, layer, material or sectiondiscussed below could be termed a second element, component, region,layer, material or section without departing from the teachings of thepresent invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “includes”,“including”, “comprises” and/or “comprising,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Embodiments of the invention are described herein with reference tocross-section illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the invention.The thickness of layers and regions in the drawings may be exaggeratedfor clarity. Additionally, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, embodiments of theinvention should not be construed as limited to the particular shapes ofregions illustrated herein but are to include deviations in shapes thatresult, for example, from manufacturing. For example, an implantedregion illustrated as a rectangle will, typically, have rounded orcurved features and/or a gradient of implant concentration at its edgesrather than a discrete change from implanted to non-implanted region.Likewise, a buried region formed by implantation may result in someimplantation in the region between the buried region and the surfacethrough which the implantation takes place. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the actual shape of a region of a device andare not intended to limit the scope of the invention.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of this specification andthe relevant art and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

Embodiments of the invention provide DMOSFET devices formed using athermally grown Metal Enhanced Oxidation (MEO) and/or high temperature(>1175° C.) NO annealing. Both processes may reduce D_(IT) near theconduction band (E_(C)), which may enable higher inversion layermobility on implanted and/or epitaxial channel devices. Furthermore, MEOand/or NO processes according to some embodiments of the invention mayexhibit controlled oxidation rates (t_(OX)˜600-900 Å) and/or temperaturestability, which may make them suitable for 4H—SiC Power MOSFETfabrication.

Referring now to FIG. 1, embodiments of a lateral MOSFET according tosome embodiments of the invention are illustrated.

As illustrated in FIG. 1, an N-channel lateral MOSFET 10 includes ap-type epitaxial layer 14 grown on a substrate 12, which may be an 8°off-axis (0001) conducting 4HP SiC crystal. Other polytypes and/oroff-axis angles of silicon carbide may also be used for the substrate12. In some embodiments, the epitaxial layer 14 may have a thickness ofabout 5 μm or more and may be formed using, for example, an MOCVDprocess, and may be doped with p-type impurities such as boron and/oraluminum at a concentration of about 5×10¹⁵-1×10¹⁶ cm⁻³. The epitaxiallayer 14 may have a thickness less than 5 μm in some cases. Inparticular embodiments, the epitaxial layer 14 may have a thickness ofabout 5 μm and may have a dopant concentration of about 5×10¹⁵ cm⁻³. Insome embodiments, the channel region of the epitaxial layer 14 may bedoped via ion implantation and may have a dopant concentration of about1×10¹⁶ cm⁻³ to 1×10¹⁹ cm⁻³.

Nitrogen and/or phosphorus ions may be implanted into the epitaxiallayer 14 to form n+ source/drain regions 16, such that the n+source/drain regions have a dopant concentration of about 1×10¹⁹ cm⁻³ ormore. However, if the dopant concentration of the source/drain regions16 is less than 1×10²⁰, a thermal anneal may be required to form ohmiccontacts thereon. In particular embodiments, the n+ source/drain regions16 are doped with phosphorus at a dopant concentration of about 1×10²⁰cm⁻³. The implants may be activated, for example by a 1650° C. Ar annealin the presence of Si overpressure. A 0.5 μm thick deposited oxide layermay serve as a field oxide (not shown). A control oxide (i.e. gateoxide) layer 18 is formed on the epitaxial layer 14 between andextending onto the source/drain regions 16. The thickness of the controloxide layer 18 may depend on the desired operating parameters of thedevice. For example, it may be appropriate to select an oxide thicknessbased on a maximum electric field of 3 MV/cm. In particular, the controloxide layer 18 may have a thickness of about 500 Å, which corresponds toa maximum gate voltage of 15 V.

The control oxide layer 18 may be grown, for example, using amulti-stage oxidation process including an oxidation step in dry O₂followed by re-oxidation (ReOx) in wet O₂ as described, for example, inU.S. Pat. No. 5,972,801, the disclosure of which is incorporated hereinby reference in its entirety.

For example, the control oxide layer 18 may be grown by a dry-wetoxidation process that includes a growth of bulk oxide in dry O₂followed by an anneal of the bulk oxide in wet O₂. As used herein,anneal of oxide in wet O₂ refers to an anneal of an oxide in an ambientcontaining both O₂ and vaporized H₂O. An additional anneal in an inertatmosphere may be performed between the dry oxide growth and the wetoxide anneal. The dry O₂ oxide growth may be performed, for example, ina quartz tube at a temperature of up to about 1200° C. in dry O₂ for atime of at least about 2.5 hours. Dry oxide growth is performed to growthe bulk oxide layer to a desired thickness. The temperature of the dryoxide growth may affect the oxide growth rate. For example, higherprocess temperatures may produce higher oxide growth rates. The maximumgrowth temperature may be dependent on the system used.

In some embodiments, the dry O₂ oxide growth may be performed at atemperature of about 1200° C. in dry O₂ for about 2.5 hours. Theresulting oxide layer may be annealed at a temperature of up to about1200° C. in an inert atmosphere. In particular, the resulting oxidelayer may be annealed at a temperature of about 1175° C. in Ar for about1 hour. The wet O₂ oxide anneal (ReOx) may be performed at a temperatureof about 950° C. or less for a time of at least about 1 hour. Thetemperature of the wet O₂ anneal may be limited to discourage furtherthermal oxide growth at the SiC/SiO₂ interface, which may introduceadditional interface states. In particular, the wet O₂ anneal may beperformed in wet O₂ at a temperature of about 950° C. for about 3 hours.The resulting control oxide layer 18 may have a thickness of about 500Å.

A gate contact 20 is formed on the control oxide 18. The gate contact 20may include, for example, boron-doped polysilicon and/or evaporatedaluminum. Boron-doped polysilicon may be used to help adjust thethreshold voltage of the device to a desired level. Polysilicon dopedwith other impurities, including n-type impurities, may also be used asa gate contact 20. In some embodiments, the thermal budget of theprocess may be a concern. In such cases, the use of evaporated aluminummay help reduce the thermal budget. Nickel source/drain contacts 22, 24may be formed on the source/drain regions 16. In particular, nickelcontacts 22, 24 formed on the n+ source/drain regions 16 may exhibitohmic behavior without any sintering.

Referring now to FIG. 2, a vertical power MOSFET 30 according to someembodiments of the invention is illustrated. Vertical silicon carbideMOSFETs are generally replicated in a unit cell. For ease ofillustration, a single unit cell vertical MOSFET will be described.

As is seen in FIG. 2, a MOSFET 30 according to embodiments of thepresent invention may include an n+ monocrystalline silicon carbidesubstrate 32. An n− silicon carbide drift layer 34 is provided on afirst face of the substrate 32. The doping and thickness of the driftlayer 34 may be determined by taking into consideration the desiredblocking voltage of the device. For example, for a high voltage device,the drift layer 34 may have a thickness of about 5 μm to about 100 μmand a doping concentration of about 8×10¹⁵ to 1×10¹⁶ cm⁻³. First andsecond implanted p-type wells 36 are formed in the n-layer 34. Thep-wells 36 may be formed using implanted aluminum, resulting in a dopantconcentration of 1×10¹⁷ to 1×10¹⁹ cm⁻³. In particular embodiments, thep-wells 36 may have a dopant concentration of about 1×10¹⁸ cm⁻³.Implanted contact regions 38 of p+ silicon carbide may also be providedin the p-wells 36. The implanted contact regions 38 may be formed, forexample, by implantation of acceptor ions, such as boron and/oraluminum, to produce regions 38 having a dopant density of about 1×10²⁰cm⁻³. In particular, implanted aluminum may be more suitable for thecontact regions 38 due to the lower diffusivity of aluminum in SiC.

First and second n+ source regions 40 are provided in the p-type wells36 adjacent the contact regions 38. The implanted source regions 40 maybe formed, for example, by implantation of donor ions to produce regions40 having a dopant density of about 1×10¹⁹ cm⁻³ or more.

An n-type silicon carbide JFET region 41 is adjacent to the n+ sourceregions 40. The JFET region 41 is spaced apart from the source regions40 by channel regions 43 in the p-type wells 36. The JFET region 41,which extends to the n-layer 34, may have a similar dopant concentrationas the drift layer 34. In some embodiments, however, the JFET region 41may be implanted with n-type impurities to have a higher dopantconcentration than the drift layer 34. For example, the JFET region 41may be implanted with donor ions to have a dopant density of from about1×10¹⁶ to about 1×10¹⁷ cm⁻³. The actual dopant concentration chosen maydepend on the desired operational characteristics of the device.

A control oxide 42 of a suitable dielectric material, such as SiO₂,extends over the JFET region 41 and the channel regions 43 to the n+source regions 40. The control oxide 42 may have a thickness of fromabout 500 to about 800 Å. In particular, the control oxide 42 may have athickness of about 500 Å.

A gate contact 46 is provided on the control oxide 42 opposite thechannel region 43. Source contacts 44 are formed on the n+ sourceregions 40. The source contacts 44 are also formed on the p+ contactregions 38 to short the n+ source regions to the p-well regions 36. Thep-well regions 36 may be relatively highly doped to reduce and/orprevent turning on the parasitic npn transistors formed by sourceregions 40, well regions 36 and drift layer 34. For example, the p-wellregions 36 may have a dopant concentration of about 1×10¹⁵ cm⁻³ to about1×10¹⁸ cm⁻³ or greater. A drain contact 48 is provided on the face ofthe substrate 32 opposite the p-type wells 36. The drain contact 48 maybe formed, for example, using nickel.

In some embodiments, the thermally grown control oxide 18 of the device10 of FIG. 1 and/or the thermally grown control oxide 42 of the device30 of FIG. 2 may be annealed in nitric oxide (NO) at a temperature aboveabout 1175° C. In some embodiments, the control oxide 18, 42 may beannealed in NO at a temperature above about 1175° C. and below atemperature at which the oxide begins to physically decompose, which maybe, for example, a temperature between about 1500° C. and about 1600° C.or above. In some embodiments, the control oxide 18, 42 may be annealedin NO at a temperature between about 1200° C. and about 1500° C. Inparticular, the control oxide 18, 42 may be annealed in NO at atemperature of about 1300° C. The anneal time may depend on the selectedanneal temperature. For example, for a 1300° C. anneal, the anneal timemay be about 2 hours.

In some embodiments, metallic impurities may be incorporated into thecontrol oxide films 18, 42 by oxidizing the channel region of thedevices 10, 30 in the presence of alumina as described, for example, incommonly assigned U.S. patent application Ser. No. 11/229,476, filedSep. 16, 2005, entitled “Methods of Fabricating Oxide Layers on SiliconCarbide Layers Utilizing Atomic Oxygen”, the disclosure of which isincorporated herein by reference in its entirety. In particular, aluminamay be provided in the oxidation chamber by mounting alumina disksadjacent wafers being oxidized. In some embodiments, the only aluminapresent in the anneal system may be the alumina disks. By limiting thealumina in the system to the alumina disks, it may be possible tocontrol and/or limit the oxidation rate.

In particular, an MEO oxidation may be performed by placing a 99.8% purealumina disk in proximity with a SiC substrate being oxidized. One wayto accomplish this is to mount an alumina disk parallel to and adjacentwith a wafer in the anneal tube. Areas of the SiC substrate that are inproximity to the alumina disk may exhibit enhanced oxidation.

Vertical MOSFET structures may also be formed using epitaxial p-typelayers as described in U.S. Pat. No. 6,653,659, the disclosure of whichis incorporated herein by reference in its entirety.

Experimental Results

The following experimental results are provided as examples only andshall not be viewed as limiting the present invention. N-channel lateralMOSFETs were fabricated as test structures on 5 μm, 5×10¹⁵ cm⁻³ p-typeepitaxial layers grown on 8° off-axis (0001) conducting 4HP substrates.Phosphorus was implanted to form the source/drain regions and a heavyaluminum dose (1×10¹⁸ cm⁻³ box profile) was implanted to simulate theDMOSFET p-well region for half of the devices (i.e. the implantedchannel devices). The implants were activated by annealing at 1650° C.in Ar with Si overpressure for about 5 minutes to about 1 hour. Asacrificial oxidation may be performed after the implants are activatedin order to improve the surface of the epitaxial layer. Furthermore, afield oxide may be grown and patterned to expose the active region ofthe device. Growth of the field oxide may also incorporate a sacrificialthermal oxidation over the active region of the device.

A 0.5 μm thick oxide layer was then deposited and patterned as the fieldoxide. In some wafers (referred to herein as the ReOx wafers), a 500 Åthick control oxide was grown at 1200° C. in dry O₂ followed by a 950°C. wet reoxidation (ReOx). Some wafers (the MEO wafers) were thermallyoxidized in the presence of metallic impurities to form a control oxidehaving a thickness t_(OX) of about 600 Å to about 900 Å. In some wafers(referred to herein as the NO wafers), the control oxide was grown usingthe reoxidation process described above and subsequently annealedin-situ in NO at 1300° C. in a silicon carbide tube coated with acoating of high quality silicon carbide deposited by chemical vapordeposition.

To form the MEO wafers, a plurality of SiC wafers to be oxidized wereplaced in SiC boats on a SiC paddle. A 99.8% pure alumina disk wasmounted upright between each of the boats, parallel to the disks in theboats. The paddle including the boats, SiC wafers and alumina disks, wasinserted into a SiC anneal tube with a loading temperature of 800° C.under a flow of N₂ and O₂. The temperature within the anneal tube wasramped up to 1000° C., and the SiC wafers were oxidized for about 6.5hours. The SiC wafers were then annealed in N₂ for about 5.5 hours at1000° C. and then the anneal tube was cooled down for two hours.

Boron doped polysilicon was deposited to form the gate electrode for theReOx and NO wafers, while evaporated aluminum was used for the MEOwafers to reduce or minimize the thermal budget. Nickel contacts wereformed on the source/drain regions by evaporation and lift-off. In theMEO wafers, the contacts were annealed at about 500° C. to make thecontacts ohmic. The anneal of MEO wafers was limited to 500° C. toprotect the aluminum gate. For the NO wafers, the contacts may beannealed at higher temperatures (e.g. about 825° C.). Companion n-typewafers were also oxidized and metallized to create NMOS-capacitors forcomparison purposes. Lateral MOSFETs were formed with a 400 μm×400 μmchannel dimension to make the channel resistance dominant for ease offield effect mobility extraction and capacitance-voltage (C-V)measurements.

Measurement of the field effect mobility of the devices was accomplishedby grounding the source contact and the backside of the wafers, andapplying a fixed voltage of 50 mV to the drain contact. The gate voltagewas swept to obtain an I_(ds)-V_(G) curve. Mobility values were thencalculated from the extracted data.

FIGS. 3A-3C show the measured NMOS C-V plotted against the theoreticalC-V curve for the ReOx, NO and MEO wafers, respectively. The ideal C-Vwas formulated to take into account the metal-semiconductor workfunction difference (φ_(MS)) and the effective fixed charge density(Q_(F)). The ReOx and MEO samples had negative Q_(F) of −1.6×10¹² cm⁻²and −6.5×10¹¹ cm⁻², respectively, which may indicate a high density ofnegatively charged midgap states that do not change occupancy during themeasurement sweep. The NO sample, on the other hand, had a positiveQ_(F) of 8.3×10¹¹ cm⁻². As illustrated in FIG. 3A, the ReOx sample alsoshowed significant stretch-out from flatband to accumulation due tointerface trapping near E_(C). The NO sample (FIG. 3B) showed markedimprovement with only a slight stretch-out, while the C-V curve of theMEO sample (FIG. 3C) was practically coincident with the theoreticalcurve.

FIG. 4 is a graph of interface state density (D_(IT)) versus energylevel from the conduction band for MEO, NO and ReOx MOSFET devices. Roomtemperature AC conductance measurements shown in FIG. 4 revealcomparably low interface state densities (D_(IT)) for both NO and MEOwafers in the measurable energy range (up to 0.2 eV below E_(C)). Thisapparent contradiction between the C-V curve and the conductance resultsmay be reconciled if the NO trap profile is increasing more rapidly thanthe MEO profile approaching the conduction band edge.

An increased oxidation rate was observed in the MEO process. Howeverthis effect may be controlled to produce an acceptable gate oxidethickness in the range of 600 to 900 Å. It is presently believed thatthe oxidation rate of MEO may be controlled/reduced bylimiting/controlling the exposure of the wafers to alumina. For example,as described above, the alumina present during oxidation was limited toalumina disks disposed adjacent the SiC wafers being oxidized.

FIG. 5 is a graph of channel mobility versus gate voltage measured atroom temperature for epitaxial channel (i.e. lateral) MEO, NO and ReOxMOSFET devices. Both the MEO and NO MOSFETs showed improved turn-oncharacteristics with peak μ_(CH) of 69 and 49 cm²/Vs, respectively. Thepeak channel mobility μ_(CH) for the MEO was roughly 50% of the valuereported in H. Olafsson, Ph.D. Dissertation, Chalmers University (2004).However, as illustrated in FIG. 6, which is a graph of channel mobilityversus gate voltage measured at various temperatures for lateral MEOMOSFET devices, the low field mobility for MEO devices increased to 160cm²/Vs at a measurement temperature of 150° C., in contrast to anirreversible 33% mobility reduction observed by Olafsson after thermalcycling. Re-measurement of the MEO MOSFET at room temperature (after thehigh temperature measurements illustrated in FIG. 6) showed a similarshape as the original room temperature curve, but shifted by a fewvolts, possibly due to mobile ion motion.

The peak channel mobility μ_(CH) for the NO wafers represents a 67%increase over the value reported by Chung, above. As shown in FIG. 7,the turn-on characteristics were only slightly diminished on theimplanted channel MOSFETs, with impressive peak channel mobilitiesμ_(CH) of 48 and 34 cm²/Vs for MEO and NO, respectively, despite the1×10¹⁸ cm⁻³ Al implant.

Olafsson also reported sensitivity of the MEO oxide to rapid thermalannealing (RTA). In order to form good power MOSFETs, it may bedesirable to utilize an RTA to sinter the ohmic contacts for lowresistance, electrical stability, and/or structural integrity. In thecase of the MEO wafers, although the particular mechanism is not wellunderstood, it is encouraging to note that the high quality MOSinterface survived a high temperature RTA with little change in theMOSFET characteristics.

In light of the improved performance of the MEO MOSFET, an investigationof the MEO oxide was performed. SIMS analysis showed a nitrogenconcentration of only 10¹⁸ cm⁻³ uniformly distributed through the oxide.This value is close to the SIMS detection limit for nitrogen and isorders of magnitude less than the mid 10²⁰ cm⁻³ concentrations typicallyrequired for effective nitrogen passivation of the 4H—SiC MOS interface.Thus, it appears that nitridation may not be the source of mobilityenhancement in the MEO wafers. The SIMS analysis results shown in FIG. 8confirm the presence of metal impurities (Fe and Cr) in highconcentrations as described in U.S. Pat. No. 6,559,068. It is noteworthythat iron was not only present in a high concentration, but that thehigh iron concentration extended all the way to the SiO₂/SiC interface.

According to embodiments of the invention, improved 4H—SiC MOSinterfaces have been obtained using both NO and MEO-based processes. AnNO anneal has been known to provide better results at highertemperatures. However, previous processes have been limited to 1175° C.due to temperature limitations of quartz furnace tubes. In someembodiments of the present invention, thermal oxidation and NO annealingin SiC tubes that are coated with SiC using chemical vapor deposition(CVD) may overcome limitations of conventional processing techniques toallow process temperatures of, for example, 1300° C. or more, which mayprovide substantial improvement over an 1175° C. NO anneal, with, insome cases, a 50% reduction in the D_(IT) at 0.2 eV below E_(C) and a67% increase in the inversion channel mobility to 49 cm²/Vs. Accordingto some embodiments of the invention, the MEO process may result in evenbetter performance with the peak channel mobility of 69 cm²/Vs at roomtemperature increasing to 160 cm²/Vs at 150° C. In MEO MOSFETs having1×10¹⁸ cm⁻³ Al-implanted channels, the mobility remains a respectable 48cm²/Vs. However, in some variations of the MEO process, it may bedesirable to remove unwanted contamination from the gate oxide that mayprovide mobile carriers at the SiO₂—SiC interface that may affectthreshold voltages.

Implanted channel MOSFET devices formed according to embodiments of theinvention (including annealing in NO at 1300° C.) and having a channeldoping of 1×10¹⁸ cm⁻³ exhibited channel mobility μ_(ch) of 35 cm²/Vs.The channel mobility of implanted channel devices is expected to belower than that of epitaxial channel devices due to implant damage.

In the drawings and specification, there have been disclosed typicalembodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

That which is claimed is:
 1. A method of forming an oxide layer onsilicon carbide, comprising: thermally growing an oxide layer in thepresence of metallic impurities on a layer of silicon carbide in asilicon carbide tube; flowing NO into the silicon carbide tube; andannealing the oxide layer in an environment containing the NO at atemperature greater than 1300° C. in the silicon carbide tube.
 2. Themethod of claim 1, wherein annealing the oxide layer comprises annealingthe oxide layer in an environment containing NO at a temperature betweenabout 1200 1300° C. and about 1600° C.
 3. The method of claim 1, whereinannealing the oxide layer comprises annealing the oxide layer for about2 hours.
 4. The method of claim 1, wherein the silicon carbide tubecomprises a tube of silicon carbide having a silicon carbide coatingthereon.
 5. The method of claim 4, wherein the silicon carbide coatingon the silicon carbide tube comprises a silicon carbide coatingdeposited by chemical vapor deposition on the silicon carbide tube. 6.The method of claim 1, wherein thermally growing the oxide comprisesthermally growing the oxide in the presence of metallic impurities. 7.The method of claim 6, wherein thermally growing the oxide comprisesthermally growing the oxide in the presence of alumina including themetallic impurities.
 8. The method of claim 1, wherein thermally growingthe oxide layer comprises thermally growing the oxide layer to athickness of between about 500 Å and 900 Å.
 9. The method of claim 1,wherein the silicon carbide layer comprises an epitaxial layer of 4Hp-type silicon carbide having an off-axis orientation that is tilted atabout 8° from a (0001) plane.
 10. The method of claim 1, whereinthermally growing the oxide layer comprises: thermally growing apreliminary oxide layer on the silicon carbide layer in dry O₂ at atemperature of about 1200° C.; and re-oxidizing the preliminary oxidelayer in wet O₂ at a temperature of about 950° C.